1. Field of the Invention
The present invention relates to a transmitter circuit that is utilized in a mobile phone and a communication device such as a wireless LAN device. More particularly, the present invention relates to a transmitter circuit that performs a polar modulation, and a communication device using the transmitter circuit.
2. Description of the Background Art
A mobile phone and a communication device such as a wireless LAN device are desired to operate with low power consumption while ensuring the precision of a transmission signal, regardless of the output power level. Therefore, adopted in the communication device is a transmitter circuit which has a small size, operates with high efficiency, and performs a polar modulation for outputting a transmission signal having high linearity.
FIG. 4 is a diagram showing a conventional transmitter circuit 900 that performs a polar modulation. In FIG. 4, the transmitter circuit 900 includes a signal generator 901, a first low-pass filter (LPF) 902, a second low-pass filter (LPF) 903, a frequency modulator 904, and an amplitude modulator 905.
Based on an input signal, the signal generator 901 generates an amplitude signal and a frequency signal. The amplitude signal is, via the first LPF 902, inputted to the amplitude modulator 905. The frequency signal is, via the second LPF 903, inputted to the frequency modulator 904.
The frequency modulator 904 frequency-modulates the inputted frequency signal, and outputs a carrier wave. The carrier wave outputted from the frequency modulator 904 is inputted to the amplitude modulator 905.
Based on the amplitude signal inputted from the signal generator 901 via the first LPF 902, the amplitude modulator 905 amplitude-modulates the carrier wave inputted from the frequency modulator 904, and then outputs the amplitude-modulated carrier wave. In this manner, the transmitter circuit 900 performs the polar modulation on the input signal, and outputs a transmission signal.
The transmitter circuit 900 performs a digital signal process on the input signal, then performs a digital-analog conversion process on the resulting signal by means of a digital-analog converter (DAC) (not shown), and finally outputs the transmission signal which is an analog signal. The transmission signal outputted from the transmitter circuit 900 includes an unnecessary noise due to the influences of a clock frequency of the DAC, an image interference frequency, a noise of an analog circuit, a digital quantization noise, and the like. The first and second LPFs 902 and 903 are provided for removal of these various noises.
FIG. 5 is a diagram showing a relationship between a cut-off frequency of the LPF and an adjacent channel leakage ratio (ACLR), and a relationship between a cut-off frequency of the LPF and a noise in a receiver band.
The ACLR is a distortion which occurs near the transmission signal outputted from the transmitter circuit 900. Rise of the ACLR causes interference with another terminal. Thus, the ACLR has to be lowered. As shown in FIG. 5, by setting the cut-off frequency of the LPF high, the rise of the ACLR can be suppressed and the influence on another terminal can be reduced.
On the other hand, when the transmitter circuit 900 is used in a mobile phone that has a reception function for receiving a signal from a base station, the transmitter circuit 900 has to reduce a noise in the receiver band so as not to influence the reception function. As shown in FIG. 5, by setting the cut-off frequency of the LPF low, the noise in the receiver band can be suppressed and the influence thereof on the reception function of the mobile phone can be reduced.
However, when the cut-off frequency of the LPF is set too low, a signal necessary for the transmission signal which will be outputted from the transmitter circuit 900 is also removed, which distorts the transmission signal and thus raises the ACLR. That is, the ACLR and the noise in the receiver band are in a tradeoff relationship.
To solve the tradeoff problem described above, it is easily conceivable to use a band elimination filter (BEF) in a frequency signal path.
FIG. 6 is a diagram showing a transmitter circuit 910 including a BEF. The transmitter circuit 910 includes a signal generator 901, a first LPF 902, a second LPF 903, a frequency modulator 904, an amplitude modulator 905, and a BEF 911. The transmitter circuit 910 shown in FIG. 6 is different from the transmitter circuit 900 shown in FIG. 4, in that the transmitter circuit 910 includes the BEF 911. For the description of the transmitter circuit 910, the same components as those of the transmitter circuit 900 shown in FIG. 4 are denoted by the same corresponding reference numerals, respectively, and the description of the components is not given.
The frequency modulator 904 frequency-modulates an inputted frequency signal, and outputs a carrier wave. Here, the carrier wave outputted from the frequency modulator 904 has a constant envelope. The BEF 911 attenuates an unnecessary frequency band, such as the reception frequency band, within the carrier wave outputted from the frequency modulator 904.
The carrier wave, in which the unnecessary frequency band has been attenuated by the BEF 911, is inputted to the amplitude modulator 905. However, the amplitude modulator 905 has no ability to sense the variation of the envelope of the carrier wave in which the unnecessary frequency band has been attenuated by the BEF 911. That is, a characteristic obtained by attenuating the unnecessary frequency band by the BEF 911 is lost.
Based on an amplitude signal inputted from the signal generator 901 via the first LPF 902, the amplitude modulator 905 amplitude-modulates the carrier wave inputted from the BEF 911, and then outputs the amplitude-modulated carrier wave. The envelope level of the carrier wave is determined based on the amplitude signal inputted from the signal generator 901 via the first LPF 902.